Constant voltage constant current control circuits and methods with improved load regulation

ABSTRACT

The present invention discloses CVCC circuits and methods with improved load regulation for an SMPS. In one embodiment, the CVCC can include: a voltage feedback circuit to generate an output voltage feedback signal; a current feedback circuit to generate an output current feedback signal; a control signal generating circuit that receives the output voltage feedback signal and the output current feedback signal, and generates a constant voltage/constant current control signal; a first enable signal generating circuit that compares a first reference voltage and the constant voltage/constant current control signal to generate a first enable signal; and a PWM controller that generates a PWM control signal based on the constant voltage/constant current control signal to control a main switch of the flyback SMPS.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No. 201210082610.1, filed on Mar. 26, 2012, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of switch mode power supply (SMPS), and more specifically to constant voltage constant current control (CVCC) circuits and methods with improved load regulation.

BACKGROUND

Switch mode power supply (SMPS) may be characterized as small volume, light weight, high power conversion efficiency, etc., and SMPS has been widely used in industrial automation, instruments and meters, medical facilities, liquid crystal display (LCD), communication apparatus, audio-visual products, digital products, and other fields. SMPS is a power supply that utilizes modern power electronics to control a ratio of a switch's on time and off time, so as to remain the output of the power supply substantially steady. Usually, electronic devices, such as MOSFETs controlled by pulse-width modulation (PWM) are used in SMPS circuits.

SUMMARY

In one embodiment, a constant voltage constant current (CVCC) circuit for a switch mode power supply (SMPS), can include: (i) a voltage feedback circuit configured to generate an output voltage feedback signal; (ii) a current feedback circuit configured to generate an output current feedback signal; (iii) a control signal generating circuit configured to receive the output voltage feedback signal and the output current feedback signal, and to generate a constant voltage/constant current control signal; (iv) a first enable signal generating circuit configured to compare a first reference voltage and the constant voltage/constant current control signal to generate a first enable signal; and (v) a pulse-width modulation (PWM) controller configured to generate a PWM control signal based on the constant voltage/constant current control signal to control a main switch of the flyback SMPS, where the PWM control signal is configured to turn on the main switch when the first enable signal is inactive, and wherein the main switch remains off when the first enable signal is active.

In one embodiment, a method can include: (i) obtaining an output current feedback signal and an output voltage feedback signal; (ii) generating a constant voltage/constant current control signal based on the output current feedback signal and the output voltage feedback signal; (iii) generating a first enable signal based on the constant voltage/constant current control signal and a first reference voltage; (iv) controlling a main switch of the flyback SMPS based on the first enable signal, where the main switch is turned off by the first enable signal when the first enable signal is active; (v) generating, in a first operation mode, a PWM control signal based on the constant current control signal to control the main switch to maintain substantially constant output current; and (vi) generating, in a second operation mode, the PWM control signal based on the constant voltage control signal, where the PWM control signal is used to control the main switch when the first enable signal is inactive.

Embodiments of the present invention can advantageously provide several advantages over conventional approaches. For example, particular embodiments can provide CVCC control circuits and methods with improved load regulation, and that have a simplified circuit structure, relatively small volume, and relatively low product cost. In addition, when under light load or even a no-load state, the main switch of the SMPS can be turned off to avoid power loss. Other advantages of the present invention may become readily apparent from the detailed description of preferred embodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an example flyback SMPS.

FIG. 2 is a block diagram of a first example power supply controlled by a CVCC control circuit in accordance with embodiments of the present invention.

FIG. 3 is a block diagram of a second example power supply controlled by a CVCC control circuit in accordance with embodiments of the present invention.

FIG. 4 is a schematic diagram of an example specific implementation of the CVCC control circuit shown in FIG. 3.

FIG. 5 is an operation waveform diagram of the CVCC control circuit shown in FIG. 4 with a light load.

FIG. 6 is an operation waveform diagram of the CVCC control circuit shown in FIG. 4 with an even lighter load.

FIG. 7 is a schematic diagram of another example specific implementation of the CVCC control circuit shown in FIG. 3.

FIG. 8 is an operation waveform diagram of the CVCC control circuit shown in FIG. 7.

FIG. 9 is another example specific implementation of the CVCC control circuit shown in FIG. 3.

FIG. 10 is a flow diagram of an example CVCC method with improved load regulation, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

Reference may now be made in detail to particular embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention may be described in conjunction with the preferred embodiments, it may be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents that may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set fourth in order to provide a thorough understanding of the present invention. However, it may be readily apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, processes, components, structures, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention.

In a common switch mode power supply (SMPS) arrangement, a flyback converter can achieve substantially constant voltage or constant current output by applying a secondary-side control circuit to control a main switch of the power circuit. With reference to FIG. 1, shown is a schematic diagram of an example flyback SMPS. The flyback SMPS can control main switch Q_(M) by secondary-side feedback control circuit 102 to realize substantially constant voltage or constant current output. For example, an output voltage signal and an output current signal of the flyback SMPS can be detected by voltage sampling resistors, current sampling resistors, and opto-coupler 101. Secondary-side feedback control circuit 102 can be used to process the detected voltage signal and current signal, and to generate a corresponding control signal to control main switch Q_(M). In this way, the flyback SMPS can generate substantially constant voltage output V_(out) or constant current output I_(out).

The constant voltage constant current (CVCC) circuit can include secondary-side feedback control circuit 102, and opto-coupler 101, used to detect the feedback signal. However, the use of opto-coupler 101 may be difficult to integrate with other circuit portions, and may thus increase overall circuit volume and product cost. Also, in many practical applications, when the load of the main circuit is reduced to a certain extent (e.g., becomes a “light” load), the primary-side circuit may still transfer energy to the output terminal, resulting in power loss.

In particular embodiments, constant voltage constant current (CVCC) control circuits and methods with improved load regulation, can include: a voltage feedback module/circuit, a current feedback module/circuit, a control signal generating module/circuit, a first enable signal generating module/circuit, and a pulse-width modulation (PWM) controller. The control signal generating module can control the flyback SMPS operating in a constant voltage or constant current state, and generate a constant voltage/constant current control signal. The PWM controller can generate a PWM control signal based on the constant voltage/constant current control signal to control the main switch. The CVCC control circuit can achieve substantially constant current control and constant voltage control through select control, so the circuit structure can be relatively simple. Also, when the example CVCC control circuit is operating under a light load or even a no-load state, the main switch can be turned off by the enable signal to stop energy from being transferred from the input side to avoid associated power loss.

In one embodiment, a CVCC circuit for a SMPS, can include: (i) a voltage feedback circuit configured to generate an output voltage feedback signal; (ii) a current feedback circuit configured to generate an output current feedback signal; (iii) a control signal generating circuit configured to receive the output voltage feedback signal and the output current feedback signal, and to generate a constant voltage/constant current control signal; (iv) a first enable signal generating circuit configured to compare a first reference voltage and the constant voltage/constant current control signal to generate a first enable signal; and (v) a PWM controller configured to generate a PWM control signal based on the constant voltage/constant current control signal to control a main switch of the flyback SMPS, where the PWM control signal is configured to turn on the main switch when the first enable signal is inactive, and wherein the main switch remains off when the first enable signal is active.

Particular embodiments can provide a CVCC control circuit with improved load regulation for a flyback SMPS. Referring now to FIG. 2, an example CVCC control circuit can include voltage feedback module 201, current feedback module 202, control signal generating module 203, first enable signal generating module 204, and PWM controller 205. As used herein, a “module” can include a circuit or circuit portion, such as a circuit integrated on an integrated circuit (IC), or a separate IC or other device, such as may be integrated on a printed-circuit board (PCB).

The CVCC control circuit can generate a corresponding control signal to control main switch Q_(M) of the power supply by processing output voltage feedback signal V_(FB) and output current feedback signal I_(FB). The operation mode of the flyback SMPS can be controlled to achieve substantially constant voltage output V_(out) and/or substantially constant current output I_(our) of the SMPS. For example, the flyback converter can include primary winding N_(p), secondary winding N_(s), and auxiliary winding N_(T). In this example, secondary winding N_(s) can be coupled to the load.

Voltage feedback module 201 can generate output voltage feedback signal V_(FB), and current feedback module 202 can generate output current feedback signal I_(FB). Control signal generating module 203 can receive output voltage feedback signal V_(FB) and output current feedback signal I_(FB), and may generate constant voltage/constant current control signal V_(comp). First enable signal generating module 204 can compare reference voltage V_(ref1) and constant voltage/constant current control signal V_(comp) to generate enable signal EN1.

PWM controller 205 can generate a PWM control signal based on constant voltage/constant current control signal V_(comp) to control main switch Q_(M) of the flyback SMPS. For example, when enable signal EN1 is inactive (e.g., a logic low level), the PWM control signal can be used to turn on or otherwise control main switch Q_(M). However, when enable signal EN1 is active (e.g., a logic high level), main switch Q_(M) may be off. In other examples, EN1 may be active to remain main switch Q_(M) on, and may be inactive to allow the PWM control signal to effectively control main switch Q_(M). The particular enable single control state and main switch on, off, or other control states, can depend on the type of main switch Q_(M) (e.g., PMOS or NMOS transistor), as well as the logic gates utilized in the control circuitry. Particular embodiments are amenable to any such type of enable-based control or enable/disable function of main switch Q_(M).

In particular embodiments, the example CVCC control circuit can control the output of the SMPS without an opto-coupler. As a result, the circuit structure can be much simpler and easier to be implemented, and the circuit volume can be relatively small as compared to conventional approaches. Also, when the operation is under a light load or even a no-load state, main switch Q_(M) can be turned off to prevent associated power loss.

Referring now to FIG. 3, shown is a specific implementation of control signal generating module 203 of CVCC control circuit. For example, control signal generating module 203 can include current controller 301 that can calculate a difference between output current feedback signal I_(FB) and reference current I_(ref1), so as to generate error signal V_(err). Voltage controller 302 can compare output voltage feedback signal V_(FB) against reference voltage V_(ref2), so as to generate control signal V_(ctrl1). Also, select controller 303 can generate constant voltage/constant current control signal V_(comp) to control the flyback SMPS to operate in a constant voltage mode or a constant current mode based on control signal V_(ctrl1) and error signal V_(err).

The following will describe example operation procedure with reference to various portions of the example circuit of FIG. 3. The flyback SMPS with substantially steady output can be controlled by the CVCC control circuit of particular embodiments, and can include signal input portion A, transformer B, load portion C, CVCC control portion D, and signal collection portion E. Signal input portion A, transformer B, load portion C, and CVCC control portion D as shown in FIG. 3 may have a similar circuit structure as the flyback SMPS discussed above.

Signal collection portion E can collect the output signals of load portion C. Specifically, auxiliary winding N_(T) coupled to secondary winding N_(S) of the flyback converter can obtain output voltage information. Also, secondary-side output voltage V_(S) obtained through dividing resistors R₁₁ and R₂₂ can be converted by voltage feedback module 201 to obtain output voltage feedback signal V_(FB). In addition, primary-side current I_(S) induced through current sense resistor R_(S) can be converted by current feedback module 202 to obtain output current feedback signal I_(FB).

Output current feedback signal I_(FB) and output voltage feedback signal V_(FB) can be input to current controller 301 and voltage controller 302, respectively. Current controller 301 can calculate a difference between output current feedback signal I_(FB) and reference current I_(ref1), to generate output error signal V_(err). Also, voltage controller 302 can compare output voltage feedback signal V_(FB) against reference voltage V_(ref2), to generate control signal V_(ctrl1).

Select controller 303 can select an operation mode of the flyback SMPS based on control signal V_(ctrl1) and error signal V_(err), and may generate constant voltage/constant current control signal V_(comp). When control signal V_(ctrl1) is at high level, the flyback SMPS may operate in a first operation mode (e.g., a constant current operation mode). When control signal V_(ctrl1) is a pulse type of signal with a certain duty cycle, the flyback SMPS may operate in a second operation mode (e.g., a constant voltage operation mode).

PWM controller 205 can generate a PWM control signal based on constant voltage/constant current control signal V_(comp) to control main switch Q_(M). For example, when enable signal EN1 is inactive, the PWM control signal can be used to turn on main switch Q_(M). Also, when enable signal EN1 is active, main switch Q_(M) may remain off, in order to reduce power loss. Enable signal EN1 can be generated by comparing constant voltage/constant current control signal V_(comp) against reference voltage V_(ref1). Enable signal EN1 can be configured to represent for a load state (e.g., light load, no load, heavy load, etc.) of the main circuit. Constant voltage/constant current control signal V_(comp) can be received by enable signal generating module 204 coupled to select controller 303. Therefore, when the load of the main circuit is reduced to a certain extent or even becomes no-load, main switch Q_(M) of the flyback SMPS can be turned off to stop energy from being transferred from the input side, and thus to avoid power loss.

When control signal V_(ctrl1) is at high level, select controller 303 can control the entire circuit operating in the first operation mode (e.g., a constant current operation mode). Select controller 303 can generate constant current control signal V_(comp) based on error signal V_(err). PWM controller 205 can receive constant current control signal V_(comp) and generate the PWM control signal to control main switch Q_(M), so as to maintain output current I_(out) as substantially constant.

When control signal V_(ctrl1) is a pulse signal with a certain duty cycle, select controller 303 can control the flyback SMPS to operate in a second operation mode (e.g., a constant voltage operation mode). Under this circumstance, select controller 303 can generate constant voltage control signal V_(comp) based on error signal V_(err). Also, PWM controller 205 can receive constant voltage control signal V_(comp) and generate the PWM control signal. For example, when enable signal EN1 is inactive, main switch Q_(M) can be turned on to maintain voltage V_(out) as substantially constant. When first enable signal EN1 is active, main switch Q_(M) may be off. That is, when the load of the main circuit is reduced to a light load or even becomes no-load, enable signal EN1 can be utilized to remain main switch Q_(M) off to stop energy from being transferred from the input side to avoid associated power loss.

In particular embodiments, a CVCC control circuit can collect voltage and current signals through signal collection portion E, and generate control signal through CVCC control portion D to control main switch Q_(M). In this way, the output voltage or current can be maintained as substantially constant with a relatively simple circuit structure also, when the CVCC control circuit is operating under a light load or even a no-load state, enable signal EN1 can be configured to turn off main switch Q_(M) to stop energy from being transferred from the input side, thus avoiding associated power loss.

Referring now to FIG. 4, shown our specific example implementations and example operation procedures of the current controller, the voltage controller, the select controller, and the first enable signal generating module. Current controller 301 of the CVCC control circuit can include transconductance amplifier 401. A non-inverting input terminal of transconductance amplifier 401 can receive reference current I_(ref1), an inverting input terminal can receive output current feedback signal I_(FB), and an output terminal can output error signal V_(err).

Voltage controller 302 of the CVCC control circuit can include comparator 402 and logic controller 403. A non-inverting input terminal of comparator 402 can receive reference voltage V_(ref2), an inverting terminal can receive output voltage feedback signal V_(FB), and an output terminal can output middle signal V_(sig). Logic controller 403 can receive middle signal V_(sig), and in one example the PWM control signal can be a clock signal. At each rising edge of the PWM control signal, control signal V_(ctrl1) can be updated as necessary based on middle signal V_(sig).

For example, at each rising edge of the PWM control signal, middle signal V_(sig) can be sampled to decide if there is a change in its detected state (e.g., detected as a high or a low level based on appropriate thresholds). If there is no change in state the detected state of middle signal V_(sig), control signal V_(ctrl1) can be maintained in its same state. However, when middle signal V_(sig) is detected or sampled as a different state, control signal V_(ctrl1) can accordingly change on a next rising edge of the PWM control signal. First logic controller 403 can be implemented by a D type flip-flop, where input terminal D can receive middle signal V_(sig), and terminal CLK can receive PWM control signal as its clock signal. Therefore, at each rising edge of the PWM control signal, the D flip-flop can effectively output middle signal V_(sig) as control signal V_(ctrl1).

Select controller 303 of the CVCC control circuit can include OR-gate 405, switch Q₁, switch Q₂, discharge circuit 406, and capacitor C₁, as shown in FIGS. 3 and 4. OR-gate 405 can receive control signal V_(ctrl1), and enable signal EN1, and can provide an output to control switch Q₁. Enable signal EN1 can be generated by enable signal generating module 204. Enable signal generating module 204 can include hysteresis comparator 404. A non-inverting input terminal of hysteresis comparator 404 can receive reference voltage V_(ref1), an inverting input terminal can receive constant voltage/constant current control signal V_(comp), and an output terminal can output enable signal EN1.

A first input terminal of switch Q₁ can receive error signal V_(err), and a second input terminal can be series connected to switch Q₂ and discharge circuit 406. For example, switch Q₂ may only be turned on when enable signal EN1 and control signal V_(ctrl1) are both inactive. Otherwise, switch Q₂ may remain off. Also, one terminal of capacitor C₁ can be coupled to a common junction of switches Q₁ and Q₂, while the other terminal can be coupled to ground. The voltage across capacitor C₁ can be configured as constant voltage/constant current control signal V_(comp). The discharging time of discharge circuit 406 can be constant or flexible. As such, discharging circuit 406 can be implemented by suitable components (e.g., a constant resistor, a variable resistor, a constant current source, or a variable current source, etc.).

FIG. 4 only shows and describes specific example implementations and operation procedure of current controller 301, voltage controller 302, select controller 303, and enable signal generating module 204. Other circuit structures like voltage feedback module 201 and current feedback module 202 are not shown. Voltage feedback module 201 and current feedback module 202 can be configured by sensors with conversion processing function and feedback function. Also, the data collection portion can have a same or a similar structure as described above, and PWM controller 205 can be configured by a PWM control circuit of a traditional SMPS.

One example operation procedure and operating principles of the CVCC control circuits in FIGS. 3 and 4 can be described below in conjunction with the signal waveform diagram shown in FIG. 5. When the main circuit is under a heavy-load condition, output voltage V_(out) may decrease. Also, output voltage feedback signal V_(FB) obtained by voltage feedback module 201 may be relatively low. Since reference voltage V_(ref2) can be larger than output voltage feedback signal V_(FB) during this time (e.g., between t₂ and t₄), middle signal V_(sig) output by comparator 402 may remain at a high level. Consequently, control signal V_(ctrl1) may also remain at a high level, and select controller 303 can control the flyback SMPS operating in the first operation mode (e.g., a constant current operation mode).

Under this circumstance, switch Q₁ may remain on and switch Q₂ may can remain off. Error signal V_(err) output by current controller 301 can be utilized to charge capacitor C₁. When the output current of the main circuit is changing, output current feedback signal I_(FB) may also change. Error signal V_(err) may increase or decrease accordingly to cause the charging current for capacitor C₁ to increase or decrease. As such, constant current control signal V_(comp) may change and this effect can be transferred to PWM controller 205. The PWM control signal generated by PWM controller 205 can control main switch Q_(M), so as to maintain the output current as substantially constant.

For example, when the main circuit is under light load conditions, control signal V_(ctrl1) can be a pulse signal with a certain duty cycle. Select controller 303 can control operation of the flyback SMPS in a second operation mode (e.g., a constant voltage operation mode). Under the constant voltage control mode, when enable signal EN1 is inactive, the PWM control signal can be configured to control main switch Q_(M) to maintain output voltage V_(out) of the flyback SMPS as substantially constant. When enable signal EN1 is active, main switch Q_(M) may remain off. In practical applications, enable control signal EN1 can be inverted by an inverter, and inversion of enable control signal EN1 and the PWM control signal can be input to two terminals of AND-gate 407. An output signal of AND-gate 407 can be configured as control signal DRV for main switch Q_(M).

In the constant voltage mode, when the load is relatively light, enable signal EN1 may remain at a low level, and the operation waveform diagram of the CVCC control circuit can be seen in FIG. 5. Since enable signal EN1 may be at a low level inactive state, switches Q₁ and Q₂ may only be controlled by control signal V_(ctrl1). During the period of t₁ to t₂, output voltage feedback signal V_(FB) can be larger than reference voltage V_(ref2), so middle signal V_(sig) generated by comparator 402 can be low. Also, control signal V_(ctrl1) can be low to control switch Q₁ to remain off, and switch Q₂ to remain on. In addition, capacitor C₁ may discharge through discharge circuit 406, and constant voltage control signal V_(comp) may decrease accordingly. Further, the duty cycle of the corresponding PWM control signal may also decrease as well as output voltage feedback signal V_(FB).

From time t₂ on, output voltage feedback signal V_(FB) may be smaller than reference voltage V_(ref2), and middle signal V_(sig) output by comparator 402 may go to a high level. By this time, since the rising edge of the PWM control signal has not yet occurred, control signal V_(ctrl1) can remain low. Thus, the switch states of switches Q₁ and Q₂ may remain the same. Also, constant voltage control signal V_(comp) can continue decreasing until time t₃. At time t₃, the rising edge of the PWM control signal can arrive, and control signal V_(ctrl1) output by D flip-flop may thus go high.

After control signal V_(ctrl1) goes to high level, switch Q₁ can be turned on and switch Q₂ can be turned off. Also, capacitor C₁ can be charged by output current feedback signal I_(FB) based on error signal V_(err). Thus, constant voltage control signal V_(comp) may increase, and the duty cycle of the PWM control signal may also increase as well as output voltage feedback signal V_(FB). By repeating such, output voltage V_(out) of the main circuit can be substantially constant.

When the load of the main circuit further decreases, enable signal EN1 can be a pulse signal. The waveform diagram of the CVCC control circuit under this circumstance can be shown as FIG. 6. As the load becomes lighter, output voltage V_(out) of the main circuit may increase. Also, output voltage feedback signal V_(FB) is relatively large and may be larger than reference voltage V_(ref2). Thus, control signal V_(ctrl1) output by comparator 402 can remain low, and the switch states of switches Q₁ and Q₂ may only be determined by enable signal EN1.

During the period of t₁˜t₂, constant voltage control signal V_(comp) may decrease to lower limit voltage V_(L) from upper limit voltage V_(H), enable signal EN1 can remain low, and the output of OR-gate 405 can also remain low. Also, switch Q₁ can remain off and switch Q₂ can remain on. Capacitor C₁ can discharge through discharge circuit 406, and constant voltage control signal V_(comp) may decrease slowly. Therefore, control signal DRV of main switch Q_(M) can be kept consistent with the PWM control signal, and output voltage feedback signal V_(FB) may increase gradually.

At time t₂, constant voltage control signal V_(comp) may decrease to lower limit voltage V_(L), and enable signal EN1 output by hysteresis comparator 404 may go high. Also, the output of OR-gate 405 may also go high to turn on switch Q₁. At the same time, switch Q₂ can be turned off. As enable signal EN1 goes high, the transconductance of transconductance amplifier 401 may decrease to generate a smaller error signal V_(err).

During the period of t₂˜t₃, capacitor C₁ can receive error signal V_(err) and can be charged. The charging time can be extended since error signal V_(err) is smaller or less than, and constant voltage control signal V_(comp) may increase gradually. Control signal DRV of main switch Q_(M) can remain low, so main switch Q_(M) can remain off, and output voltage feedback signal V_(FB) can decrease gradually.

The illustrated implementations of all the example modules or circuits can be replaced by other circuit structures or other components with the same or a similar function. For example, the switches of the CVCC control circuit can be configured by MOSFET transistors, or any other appropriate transistors or switching devices. The CVCC control circuit of particular embodiment can control the circuit operation mode by applying a select controller. When the load of the main circuit is reduced to a certain predetermined level, enable signal EN1 can maintain main switch Q_(M) at an off state to stop energy from being transferred from the input side of the main, thus avoiding associated power loss.

In particular embodiments, output voltage feedback signal V_(FB) can decrease to reference voltage V_(ref2) when constant voltage control signal V_(comp) rises to upper limit voltage V_(H). However, if the load becomes lighter, and possibly becomes no-load, output voltage feedback signal V_(FB) may not reduce to reference voltage V_(ref2). Yet when constant voltage control signal V_(comp) has risen to upper limit voltage V_(H) and enable signal EN1 has gone low, output voltage feedback signal V_(FB) may start to gradually increase as control signal DRV of main switch Q_(M) can remain consistent with the PWM control signal. In order to extend the off time of main switch Q_(M) when the main circuit is under a light load or no-load state, adjustments on the CVCC control circuit described above can be made, as will be discussed further below with reference to FIG. 7.

Referring now to FIG. 7, compared to the above described example (see, e.g., FIG. 4), the present example CVCC can convert two-input AND-gate 407 to three-input AND-gate 704. Also, two input signals of three-input AND-gate 704 can remain unchanged to be an inverted version of enable signal EN1, and the PWM control signal. The third input terminal of AND-gate 704 can receive enable signal EN2 generated by enable signal generating module 720. Enable signal generating module 720 can include comparator 701, AND-gate 702 and RS flip-flop 703, as shown in FIG. 7.

A non-inverting input terminal of comparator 701 can receive output voltage feedback signal V_(FB), an inverting input terminal can receive reference voltage V_(ref3), and an output terminal can be coupled to a first input terminal of AND-gate 702. A second input terminal of first AND-gate 702 can receive an inverted version of the PWM control signal, and an output terminal can be coupled to a reset terminal of RS flip-flop 703. A set terminal of RS flip-flop 703 can receive enable signal EN1, and an output terminal can output enable signal EN2.

When enable signal EN2 is active high and enable signal EN1 is inactive low, the PWM control signal can be used to control main switch Q_(M). When enable signal EN2 is inactive low or enable signal EN1 is active high, main switch Q_(M) may remain off. As shown in FIG. 7, enable signal EN1 can be used to obtain an inverted version of signal enable signal EN1 through an inverter. The inverted version of enable signal, enable signal EN2, and the PWM control signal can be input to terminals of AND-gate 704. The output signal of AND-gate 704 can be configured as control signal DRV for main switch Q_(M).

Example operation procedure and operation principles of the example CVCC control circuit of FIG. 7 can be described in detail below in conjunction with the specific waveform diagram. Referring now to FIG. 8, during the period of t₁˜t₂, output voltage feedback signal V_(FB) may be smaller than reference voltage V_(ref3), so the output signal of second comparator 701 can be at a low level to cause enable signal EN2 to go high. During this period, the operation procedure and operation principle of the CVCC control circuit can be substantially the same as the CVCC control circuit shown in FIG. 4.

When the load of the main circuit is even lighter than in the example operation discussed above, or even becomes no-load, output voltage V_(out) of the main circuit may increase to cause output voltage feedback signal V_(FB) to not fully decrease to second reference voltage V_(ref2) by time t₂. However, at time t₂, constant voltage control signal V_(comp) may already have risen to upper limit voltage V_(H) to result in enable signal EN1 going low. Thus, the PWM control signal can still control main switch Q_(M). From time t₂ on, output voltage feedback signal V_(FB) can start rising, and constant voltage control signal V_(comp) may decrease due to the discharging of capacitor C₁.

At time t₃, output voltage feedback signal V_(FB) may have risen to reference voltage V_(ref3), so the output signal of comparator 701 may go high and be input to first AND-gate 702. At the same time, PWM control signal may be at a low level, AND-gate 702 can generate a high signal to the reset terminal of RS flip-flop 703 to reset enable signal EN2 to low. From time t₃ on, control signal DRV of main switch Q_(M) may go low to turn off main switch Q_(M). Also, output voltage feedback signal V_(FB) may decrease gradually as the energy transferred from the input side of the main circuit is stopped.

During the period of t₃˜t₄, enable signal EN2 can stay at a low level. Since enable signal EN1 remains at a low level, the discharging of capacitor C₁ may cause constant voltage control signal V_(comp) to continue to decrease. At time t₄, constant voltage control signal V_(comp) can decrease to lower limit voltage V_(L) to cause enable signal EN1 to go high. Because enable signal EN1 can be input to the set terminal of RS flip-flop 703, enable signal EN2 output by RS flip-flop 703 can go high.

During the period of time t₄˜t₅, as enable signal EN1 goes high, main switch Q_(M) can remain off. Also, output voltage feedback signal V_(FB) may continue to decrease, capacitor C₁ may begin to charge, and constant voltage control signal V_(comp) may start rising until reaching upper limit voltage V_(H) at time t₅. When the load of the main circuit is reduced to a certain extent or even becomes no-load, the example CVCC control circuit can turn off main switch Q_(M) by enable signal EN2 to stop energy from being transferred from the input side, thus avoiding associated power loss.

In particular embodiments, the output voltage signal can be collected through auxiliary winding N_(T) to obtain output voltage feedback signal V_(FB). Since output voltage V_(s) of auxiliary winding N_(T) may be a discontinuous signal, output voltage feedback signal V_(FB) may be pulled down to avoid the entire circuit possibly suffering from undesirable effects due to signal discontinuity, such as entering a locked state. In view of this, particular embodiments can also provide another specific implementation of the CVCC control circuit, as will be discussed below with reference to FIG. 9.

Referring now to FIG. 9, the example CVCC control circuit can also include AND-gate 901 and recovery circuit 902, as compared to the above example. AND-gate 901 can receive control signal V_(ctrl1) and enable signal EN2. Also, an output terminal of AND-gate 901 can connect to an input terminal of OR-gate 405 of select controller 303. When the output of comparator 701 is high, recovery circuit 902 can pull down output voltage feedback signal V_(FB). As a result, the output of comparator 701 can quickly recover to a low state.

In this example, recovery circuit 902 can include switches Q₃ and Q₄. A first terminal of switch Q₃ can connect to the non-inverting input terminal of comparator 701. A second terminal of switch Q₃ can be series connected with switch Q₄ to ground. Also, a control terminal of switch Q₃ can connect to a first input terminal thereof, in a diode-connected transistor configuration. A control terminal of switch Q₄ can connect to the output terminal of comparator 701.

When output voltage feedback signal V_(FB) is larger than reference voltage V_(ref3), the output of comparator 701 can go high to turn on switch Q₄. Thus, output voltage feedback signal V_(FB) can be pulled down by switches Q₃ and Q₄ to result in the output of comparator 701 going low again. Therefore, when output voltage feedback signal V_(FB) is larger than reference voltage V_(ref3), the output of comparator 701 can be a single pulse signal.

At the same time, since output voltage feedback signal V_(FB) can be pulled down, it may be smaller than reference voltage V_(ref2). Control signal V_(ctrl1) output by voltage controller 302 can go high to ensure that switch Q₁ cannot be turned on by mistake. Also, switch Q₂ cannot be turned off by mistake. Control signal V_(ctrl1) and enable signal EN2 can be coupled to the input terminal of AND-gate 901. When control signal V_(ctrl1) goes high because output voltage feedback signal V_(FB) is pulled down, comparator 701 can output a low level to keep enable signal EN2 at a low level. Thus, the output of AND-gate 901 can remain inactive low, and may be input to OR-gate 405.

In particular embodiments, output voltage feedback signal V_(FB) can be pulled down via AND-gate 901 and recovery circuit 902 to avoid the entire circuit entering a locked state. In addition, when the output of the SMPS is controlled as substantially steady, main switch Q_(M) can be controlled along with the load change to avoid power losses, as discussed above.

In one embodiment, a method can include: (i) obtaining an output current feedback signal and an output voltage feedback signal; (ii) generating a constant voltage/constant current control signal based on the output current feedback signal and the output voltage feedback signal; (iii) generating a first enable signal based on the constant voltage/constant current control signal and a first reference voltage; (iv) controlling a main switch of the flyback SMPS based on the first enable signal, where the main switch is turned off by the first enable signal when the first enable signal is active; (v) generating, in a first operation mode, a PWM control signal based on the constant current control signal to control the main switch to maintain substantially constant output current; and (vi) generating, in a second operation mode, the PWM control signal based on the constant voltage control signal, where the PWM control signal is used to control the main switch when the first enable signal is inactive.

Referring now to FIG. 10, particular embodiments can provide a CVCC control method with improved load regulation for flyback SMPS. For example, the CVCC control method can include obtaining output current feedback and output voltage feedback signals at S1001. For example, a signal collection circuit can be configured to collect output current and output voltage signals of the SMPS. Also, voltage and current feedback modules can process the output current and voltage signals to obtain output current and voltage feedback signals.

At S1002, a constant voltage/constant current control signal can be generated based on the output current feedback signal and the output voltage feedback signal. At S1003, an enable signal can be generated based on the constant voltage/constant current control signal and a reference voltage. At S1004, a main switch of the flyback SMPS can be controlled based on the enable signal.

At S1005, in the first operation mode, a PWM control signal under constant current state can be generated based on the constant current control signal to control the main switch, so as to remain the output current as substantially constant. At S1006, in the second operation mode, a PWM control signal under constant voltage state can be generated based on the constant voltage control signal. If the enable signal is active, the main switch can be turned off by the enable signal. If the enable signal is inactive, the main switch can be controlled (e.g., turned off/on) by the PWM control signal. Further, the enable signal can be configured to represent a load state of the main circuit. For example, when under light load or no-load condition, the main switch can be turned off to stop energy from being transferred from the input side, thus avoiding associated power loss.

In addition, the above described control method can also include generating an error signal and a control signal based on the output current feedback signal and the output voltage feedback signal, and calculating the difference between the output current feedback signal and a reference current to obtain the error signal. Also, the output voltage feedback signal can be compared against a second reference voltage to generate the control signal. Also, the operation mode of the flyback SMPS can be selected based on the error signal and the control signal.

When the SMPS is working operates in the second operation mode, the above described control method can also include obtaining a second enable signal based on the output voltage feedback signal, a third reference voltage, the PWM control signal, and the first enable signal. When the second enable signal is active and the first enable signal is inactive, the main switch can be controlled based on the PWM control signal. When the second enable signal is inactive or the first enable signal is active, the main switch can be turned off.

The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents. 

What is claimed is:
 1. A constant voltage constant current (CVCC) control circuit for load regulation for a flyback switch mode power supply (SMPS), the CVCC comprising: a) a voltage feedback circuit configured to generate an output voltage feedback signal; b) a current feedback circuit configured to generate an output current feedback signal; c) a control signal generating circuit configured to receive said output voltage feedback signal and said output current feedback signal, and to generate a constant voltage/constant current control signal; d) a first enable signal generating circuit configured to compare a first reference voltage and said constant voltage/constant current control signal to generate a first enable signal; and e) a pulse-width modulation (PWM) controller configured to generate a PWM control signal based on said constant voltage/constant current control signal to control a main switch of said flyback SMPS, wherein said PWM control signal is configured to turn on said main switch when said first enable signal is inactive, and wherein said main switch remains off when said first enable signal is active.
 2. The CVCC control circuit of claim 1, wherein, said control signal generating module comprises: a) a current controller configured to calculate a difference between said output current feedback signal and a first reference current to generate an error signal; b) a voltage controller configured to compare said output voltage feedback signal against a second reference voltage to generate a first control signal; and c) a select controller configured to control whether said flyback SMPS operates in a constant voltage mode or a constant current mode based on said first control signal and said error signal, and to generate said constant voltage/constant current control signal.
 3. The CVCC control circuit of claim 1, wherein said first enable signal generating module comprises a first hysteresis comparator having a non-inverting input terminal configured to receive said first reference voltage, an inverting input terminal configured to receive said constant voltage/constant current control signal, and an output terminal configured to output said first enable signal.
 4. The CVCC control circuit of claim 2, wherein said voltage controller comprises: a) a first comparator having a non-inverting input terminal configured to receive said second reference voltage, an inverting input terminal configured to receive said output voltage feedback signal, and an output terminal configured to output a middle signal; and b) a first logic controller configured to receive said middle signal, and to generate said first control signal, wherein said PWM control signal is used as a clock signal for said first logic controller.
 5. The CVCC control circuit of claim 2, wherein said select controller comprises: a) an OR-gate configured to receive said first control signal and said first enable signal; b) a first switch, wherein a control terminal of said first switch is coupled to an output terminal of said first OR-gate, and a first terminal of said first switch is configured to receive said error signal; c) a second switch coupled to said first switch and a discharge circuit, wherein said second switch is configured to turn on when said first enable signal and said first control signal are inactive; and d) a capacitor coupled to ground and a common junction of said first switch and said second switch, wherein a voltage across said first capacitor is configured as said constant voltage/constant current control signal.
 6. The CVCC control circuit of claim 1, further comprising a second enable signal generating module having: a) a second comparator configured to receive said output voltage feedback signal and a third reference voltage; b) a first AND-gate coupled to receive an output of said second comparator and an inverted version of said PWM control signal; and c) an RS flip-flop, wherein a reset terminal of said RS flip-flop is coupled to an output of said first AND-gate, a set terminal of said RS flip-flop is coupled to said first enable signal, and an output terminal of said RS flip-flop is configured to output a second enable signal, d) wherein said PWM control signal is configured to turn on said main switch when said second enable signal is active and said first enable signal is inactive, and e) wherein said main switch remains off when said second enable signal is inactive or said first enable signal is active.
 7. The CVCC control circuit of claim 6, further comprising: a) a second AND-gate configured to receive said first control signal and said second enable signal, and having an output coupled to said control signal generating module; and b) a recovery circuit configured to pull down said output voltage feedback signal to fast recover an output of said second comparator to a low level when said output of said second comparator is active.
 8. A constant voltage constant current (CVCC) control method for load regulation for a flyback switch mode power supply (SMPS), the method comprising: a) obtaining an output current feedback signal and an output voltage feedback signal; b) generating a constant voltage/constant current control signal based on said output current feedback signal and said output voltage feedback signal; c) generating a first enable signal based on said constant voltage/constant current control signal and a first reference voltage; d) controlling a main switch of said flyback SMPS based on said first enable signal, wherein said main switch is turned off by said first enable signal when said first enable signal is active; e) generating, in a first operation mode, a pulse-width modulation (PWM) control signal based on said constant current control signal to control said main switch to maintain substantially constant output current; and f) generating, in a second operation mode, said PWM control signal based on said constant voltage control signal, wherein said PWM control signal is used to control said main switch when said first enable signal is inactive.
 9. The method of claim 8, further comprising: a) generating an error signal by calculating a difference between said output current feedback signal and a first reference current; b) comparing said output voltage feedback signal against a second reference voltage to generate said first control signal; and c) selecting one of said first and second operation modes of said flyback SMPS based on said error signal and said first control signal.
 10. The method of claim 8, further comprising: a) obtaining a second enable signal based on said output voltage feedback signal, a third reference voltage, said PWM control signal, and said first enable signal; b) controlling said main switch based on said PWM control signal when said second enable signal is active and said first enable signal is inactive; and c) turning off said main switch when said second enable signal is inactive or said first enable signal is active. 